This book is such a fine mix of reflecton, anecdote and quote, so nicely paced and salted, one wishes foolishly at its end to begin again -- with a new spring, another glimpse of seal and tern, a whole new series of excursions in fog and flat calm and starry night.

Pleasant Bay, the subject of "Shallow Waters," is the large estuary at the elbow of Cape Cod, Mass. From breachings of the dunes at Nauset Beach it is becoming shallower, marshier and in the process ever more biologically productive, "a boater's nemesis but a naturalist's delight."

Boy and man, William Sargent has monitored the bay's charges from a family house on its shores. One brown bag lunch with a batch of scrapping malacologists -- "loud, boisterous, witty, opinionated" -- convinced him that his quest to understand this body of water, to find how animals evolve behaviors to harvest its riches, could be his career.

Underwater, the harbingers of spring are teeming acres of plnkton. Alewives leap up the first step of the fish ladder. On May 5 the striped bass arrive from the Chesapeake Bay -- schoolies in the lead, then adults, last among them the bulls (females which may be as old as 40 years).

In the full moon of July's highest tide, Sargent attends a horseshoe crab orgy where hundreds of crabs, their shells scraping and scratching, dig into the sand to lay and fertilize eggs just as the tide turns. As a boy, in the Cape Cod tradition, he has dispatched his share of the ugly armor-plated creatures. And he has watched three generations of Sargent dogs drag crabs by their spikey tails to burials above the high-tide line. Today the Paleozoic crab is prized for its blue blood, a remarkable tool for diagnosing spinal meningitis. Processed, the blood sells for $15,000 a quart. d

Ruddy turnstones, small shore birds, appear in this book rooting through the marsh like feathered pigs; and a pair of gulls with access to a high-class dump, their nests surrounded by the scarlet shells of cooked lobster. A green heron, fishing, drops a blade of grass into the current and strikes out with its snakelike neck to capture a minnow as it rises to his lure. On a visit to a tern colony, Sargent spends a day doubled over, to avoid raps on the head from parent birds while he scrutinizes their nestlings.

There's a splendid Indian summer chapter on Cape Codders out digging quahogs and catching blue crabs for their freezers. Experienced rakers, it seems, jerking their 15-foot-long rakes across the bottom can tell if the teeth are hitting little necks or cherry stones or chowders -- the three market sizes that determine pay.

Sargent looks back in passing to when oystering was good, and turtling, and to a time when the bay island supported cattle and the horses wore wooden shoes for harvesting marsh hay.

One day his boat goes aground, and he waits out the tide in talk with a fisherwoman, similarly stranded. From paths through the eelgrass he notices that eels have started their migration to the Sargasso Sea. There they'll spawn the transparent offspring that are swept back north by the Gulf Stream, metamorphosed by the time they reach the bay into elvers capable of swimming -- "tiny vital threads of life."

By winter Sargent has looked in on a colony of island seals and sensibly made his way in from a storm to a fireside talk of rescues at sea.

If you open this book to the blue eyes of the blue-eyed scallop, don't be misled into thinking this is another book on curious creatures of the sea with a patched-together text. It's not. There happens to be some showoff marine life, but most of the photographs are by the author, who thinks nothing of lying about for months in six inches of creek water to catch with his hand-held camera just the animal behavior he wants.